Register Now

Login

Lost Password

Lost your password? Please enter your email address. You will receive a link and will create a new password via email.

222-9+11+12:2*14+14 = ? ( )

Toán Lớp 8: a ) ( a+b ) ^3 + ( a – b ) ^3 = 2a ( a^2 + 3b^2) b) ( a + b ) ^3 – ( a – b )^3 = 2b*(b^2+3a^2) mk ms học lm cách dễ nhé đừng lm mấy các

Toán Lớp 8: a ) ( a+b ) ^3 + ( a – b ) ^3 = 2a ( a^2 + 3b^2)
b) ( a + b ) ^3 – ( a – b )^3 = 2b*(b^2+3a^2)
mk ms học lm cách dễ nhé đừng lm mấy cách khó

Comments ( 2 )

  1. a)(a+b)^{3}+(a-b)^{3}=2a(a^{2}+3b^{2})
    ↔[(a+b)+(a-b)][(a+b)^{2}-(a+b)(a-b)+(a-b)^{2}]=2a^{3}+6ab^{2}
    ↔(a+b+a-b)(a^{2}+2ab+b^{2}-a^{2}+b^{2}+a^{2}-2ab+b^{2})=2a^{3}+6ab^{2} ↔2a(a^{2}+3b^{2})=2a^{3}+6ab^{2}
    ↔2a^{3}+6ab^{2}=2a^{3}+6ab^{2}
    ⇒VT=2a^{3}+6ab^{2}
        VP=2a^{3}+6ab^{2}
    Vậy VT=VP
    ⇒(a+b)^{3}+(a-b)^{3}=2a(a^{2}+3b^{2}) (đpcm)
    b)(a+b)^{3}-(a-b)^{3}=2b(b^{2}+3a^{2})
    ↔[(a+b)-(a-b)][(a+b)^{2}+(a+b)(a-b)+(a-b)^{2}]=2b^{3}+6a^{2}b
    ↔(a+b-a+b)(a^{2}+2ab+b^{2}+a^{2}-b^{2}+a^{2}-2ab+b^{2})=2b^{3}+6a^{2}b ↔2b(3a^{2}+b^{2})=2b^{3}+6a^{2}b
    ↔2b^{3}+6a^{2}b=2b^{3}+6a^{2}b
    ⇒VT=2b^{3}+6a^{2}b
        VP=2b^{3}+6a^{2}b
    Vậy VT=VP
    ⇒(a+b)^{3}-(a-b)^{3}=2b(b^{2}+3a^{2})(đpcm)
     

  2. a) (a+b)^3+(a-b)^3
    =a^3+3a^2b+3ab^2+b^3+a^3-3a^2b+3ab^2-b^3
    =2a^3+6ab^2
    =2a(a^2+3b^2) (đpcm)
    b) (a+b)^3-(a-b)^3
    =a^3+3a^2b+3ab^2+b^3-(a^3-3a^2b+3ab^2-b^3)
    =a^3+3a^2b+3ab^2+b^3-a^3+3a^2b-3ab^2+b^3
    =6a^2b+2b^3
    =2b(3a^2+b^2) (đpcm)

Leave a reply

222-9+11+12:2*14+14 = ? ( )